Step 14: Final Comments

Step 15: FAQ & Design Tradeoffs

FAQ's 1) Do I really need resistors?Absolutely, Yes.It's a very bad idea to connect active outputs together without some resistance in between.Depen...

My car stereo has only one 'Aux' input, but I have several gadgets that I like to hear on a trip without swapping cables: MP3 Player, talking GPS, Blackberry, XM player, Kindle, etc.

I couldn't find a commercial solution, so I came up with this little passive audio mixer that takes 1/8" headphone outputs from up to four devices and safely mixes the signals into one line that can feed a car stereo Aux input, powered speaker or any other amplifier that can take 1/8" stereo output.

No batteries are required, it's sonically transparent and most audio gadgets have their own volume control so the mixer can be done very simply and cheaply.

This little mixer also works great for connecting multiple computers to one set of amplified speakers and has many other possible uses. Note that this device is NOT RECOMMENDED FOR HEADPHONES!!!! Whatever you plug into the output needs to have its own amplification, or the volume will probably be too low.

Note: Soldering is required. If you don't know how, please search for Instructables on how to solder, as that's beyond the scope of this project.

Note 1: A normal Altoids Tin can take 4 or more inputs, this example assumes 4 inputs. The Altoids Gum tins can fit 3 inputs comfortably. Adjust the number of jacks according to:

# of Jacks = 1+ # of inputs

and number of resistors by:

# of Resistors = 2 * # of inputs

Note 2: All resistors (2 per input channel) should have the same value, which can be anything between 1k and 10k ohms. Higher values result in a higher volume drop. Also, the resistor power rating can be 1/8 or higher. This circuit runs no power through it, so there's no need for bigger than 1/8 w, but use whatever you have handy. Bigger power rating = bigger size.

This was exactly what I was looking for. Thank you for making the instructions clear and actually including a circuit diagram that wasn't made in MS Paint. It works perfectly for what I need, and every input comes out the other end crystal clear, even when multiple sources are playing at once.

I ended up using a Radioshack project box instead of an Altoids tin, mostly because I just didn't have one sitting around and the box was actually cheaper than a tin of Altoids. This also makes the drilling significantly easier and prevents any unintentional shorts.

Thank you again for taking the time to make this useful, clear and concise guide.

Looks like a 2-sided board with the silkscreen mask drawn in so you would either need the file or just reproduce it on your own. It isn't mine and I don't have it. Maybe chipmonger would nice enough to provide if you asked.

C3-PO is an old ball mouse. After removing the guts there was plenty of room for the wiring. For the triple input audio jack I used parts from an old motherboard's sound card. They have five leads on each; 1) ground, 2) closed tip, 3) open tip, 4) closed ring, 5) open ring. When a plug is inserted, the part that touches the tip moves from the open tip lead to the closed tip lead. This must be how a computer knows when a plug is inserted. I used the closed tip/ring leads, leaving the open/tip leads alone. I wonder if the the open leads could be used for anything.

Thank you for this thread! I've spent a while Googling for answers to questions I had regarding multiple signals, risks and protection!

Many people think it's risk free using Y-cables with adapters or crossing connections unprotected which to me is just bonkers!

Being new to audio I was just going to use diodes so I appreciate you explaining the futility of this method!

I have a question regarding values!

You specify 1k to 10k and from what I can see from your photos you selected 4.7k, does this have specific advantages? I only ask as previous searches, 4.7k and 10k seem to be popular choices with audio mixers!

Hello! Thank you so much for getting back to me and so quickly too! Really appreciate your help with this, I found so many other threads on other sites with bad info! I found your instructions very educational and really helpful!

I need a passive mixer to combine two preouts from an amplifier to feed a powered 2.1 speaker set with only one input. Would 2k resistors be appropriate for two input sources carrying 1v with 1.2k impedance?

What you really want is a headphone splitter (which you can buy very cheaply) but if you want to build your own (and you don't ever plan to mix active signals together), then yes. But in this case, I'd recommend building without the resistors, because they'll reduce your headphone volume and provide no benefit.

Hello rich I just checked out your projects last night thank you for your detailed instructions with pictures , I just want to know if I use the output jack as an input eg. Plug my phone using aux cable can I plug 4 headphones and listen to them through the other 4 jacks , just want to find out bro thanks

Thanks a bunch for the knowledge. I built my version with an aluminum RadioShack project box because all I could find were 1/4" stereo jacks. I used Sugru to solder and added a 5th input. I even had to 'stretch' a resistor using some of the solid wire because my placement was a bit spaced and all is well. One lovely solution to mix my family of Korg devices. Do it folks!

Hi :) Love your tutorial and decided to build it myself with 3 inputd and also added an audio-taper 10kOhm pot master volume before the output jack. But I have ran into a problem. When I tested my circuit after soldering the first of the three inputs I noticed that the right channel is FAR more quiet than the left. The pot shouldnt be crappy quality (ie. the tolerances between the gangs shouldnt be too much). Can I fix it by putting a smaller value resistor for the left channel output ?

Thanks for the heads up, Im going to try to put different value resistor for the quieter channel to raise its volume. I've tried earlier to measure the resistance between the two channels and the difference is about 0.2-0.5kOhm

As it stands, a mic wouldn't work, but with a few mods it could happen. What are you trying to do exactly? Have 1 mic send audio to 3 devices at once?

I did a variation so my son and I could play minecraft together and record our (hilarious) commentary from 2 headset mics along with the game sounds -- worked well, but not quite what you're looking for.

Also, I'm thinking of doing an instructable for a pc gaming headset mixer that would combine game sound, incoming voice chat from game/skype/teamspeak and outgoing mic from 1 or more mics/headsets. This will require power and active amplification. Stay tuned on that.

Thanks for your reply, will def be keeping a lookout for your next Guide. After some consideration I now believe a simple switch to choose which output the mic gets sent to is better than mixing it, rather than a constant feed to all devices.

On a diff note, I'm already talking to a couple of CMoy builders on making a a setup for me which includes your passive mixer which I will use to mix my laptop, iphone, ps4 outputs to 1 pair of earphones.

And I'll get 1 mic make a switch to direct it to my laptop, iphone, ps4.

Great article great project.i read through all the comments but couldn't find an answer to my following question....I'm trying to connect an external bluetooth for my BlackBerry through the aux input on my car radio and recently I'm trying to add an external xm radio through the same input. I've tried using a dpdt switch to disconnect the 2 sources (I have all the grounds tied together....but I still get a bit of feedback when I switch to the BT. XM works great! Any ideas?

try changing things around, one thing at a time, while paying attention to conditions when the noise appears and disappears. Swap cables, remove mixer altogether and direct connect, etc. This will help eliminate variables.

What Bluetooth device (receiver?) are you using and how is it powered? Is it really feedback or just noise? (trend is to use these terms interchangeably, but they're very different!)

As soon as a charger/power supply is connected to most devices, audible (and annoying) noise is introduced into the audio. It's because the switching converters used to go from +12V car power to +5V USB are *very* noisy and hard to filter out. If your Bluetooth device has a battery option, try that and see if the noise goes away.

Proximity to the mobile phone itself can introduce noise, e.g. if my mobile phone is next to my office desk phone, audible noise in present, but it tends to come and go.

Good luck, this type of thing can be tough to get rid of, or you might get lucky and find the cause.

I'm using a Kinivo BTC450 Bluetooth Hands-Free Car Kit for Cars with Aux Input Jack .... no noise when plugged in directly and I noticed minimal noise with the car in the ACC position. Sometimes it's there ... sometimes not when I have the 2 units plugged in ... the power source (the 12 volt plugs) may not be the problem - it could be a shielding issue from the aux input (located in the arm rest) to the radio. I did come across this unit (saw it on Amazon) : PAC Ground Loop Isolator for 3.5 Mm Applications - what are your thoughts on that? They want $17.00 on line ... can probably build on for a lot less. Tnx for your feedback (pun not intended!)

cool set up. i'm trying to do something similar, but different. I am a hang glider pilot and take in flight videos. I also have a ham radio that I use to talk to my chase driver. I have a mic and speakers wired into my helmet with a three conductor jack that has mic, speaker and ground connections. The mic is keyed with a PTT switch that runs down my sleeve.

What I would like to do is be able to record the radio transmissions in both directions by splitting out the mic and speaker wires to a second jack that I can connect to a voice recorder. What would be a real plus is if I could also use the mic to narrate when I am not transmitting. Any thoughts from anyone???? Thanks

You'll need 2 splits and only one mixer to mix radio output (your headphone line) with your voice input (your mic line) onto the output, that connects to camcorder input. However you'd need a mini mic-amplifier, probably battery powered, since your voice from mic is only amplified in radio and too low for camcorder. Mixer assembly you'd plug in BEFORE your PTT switch so that voiceover is mixed to video at all times while only broadcast to radio when switch is pushed. See little scematic I drew for you (sorry for Paint :) )

I was looking for something simple, This is all over my head. I need something that will take input from a small MP3 player and a microphone on my collar and output to a speaker. I want to be able to sing along with a karaoke track and amplify it so it can be heard through the speakers. Can anyone help?

Are you looking for wired or wireless? Do you have equipment already? (e.g. lapel mic, PA system, etc) What's your budget? What quality do you need?(i.e. home use or pro-quality for paid gigs?). Can you solder? Do you want to build or buy something? I could make suggestions, but they would be just a guess without knowing more.

FYI, the little mixer in this instructable combines stereo low-power signals of similar strength into one stereo signal. It doesn't do any amplification (in fact it reduces the volume a bit), so I don't think it's the right thing for your application.

Yes, of course. Use audio taper slide pots and there's a schematic for volume control with potentiometers in the FAQ portion. Probably tricky to do in an altoids tin though. Both getting small enough slides and cutting slots in the flimsy metal without making a mess will be a challenge. Good luck.

My previous post was confusing so I deleted it. I'm wondering if I could mix 1/4" inputs with 1/8" and if so, in what order. I'm also wondering if it'd be safe to connect all input jacks to a 1/8" output that also flows to a 1/4" output. I would only connect anything to one of the two outputs at a time.

The size of the connector isn't important at all. What *is* important is the electrical power of the signals you're mixing. As long as all the signals are of a similar voltage/power range, you should be fine and connector size/shape doesn't matter in the least.

OTOH, if you are trying to mix a weak signal (say, directly from a microphone or electric guitar) with a strong signal (say, from a headphone output of a smartphone, etc.) the weak signal will be totally lost.

Yeah, I think so. I plan on using it for an electric piano at school with a battery powered metronome and at home to mix a TV and PC via headphone jacks. I'll probably use three 1/8" and one 1/4". Now I just need to learn how to solder and buy a soldering iron.

First off, kudos for trying to "do the math" -- that said, you *really* don't have to. Just use resistors in the neighborhood of 1K Ohms (really from a few hundred on up) and you'll be fine.

Also note that If you just want a splitter, you don't need resistors at all, and you can buy commercial devices cheaply. The resistors are necessary when you want to *combine* multiple signals into one.

So now I'm going to correct a few things in case you really do want to learn something (not trying to be mean at all):

1) four 1k resistors in series (connected end to end) have a value of 4k ohms. four 1k resistors in parallel (resistors side-by-side) have a value of 250 ohms. Try googling 'parallel resistor equation' for a good explanation.

2) Not sure where you're getting a Zin of 5 ohms. This would mean you're trying to drive a passive speaker and that is not going to work with a passive mixer (well covered in the instructable and many discussions). Most powered speakers, preamps, etc have Zin values from 1K on up to 10 or 100k ohms.

3) not sure where you're getting the equation you have written out, but it doesn't make any sense to me. When you compute a parallel resistance value, you have to be very careful of where the parentheses go or you'll end up with wacky numbers -- maybe that's what's happened here?

Swapped the input/output. The output is the speaker, in the input of coarse the iPod, computer etc. My speakers have a impedance at >5k just forgot the k, 5000 not 5. But no need to calculate this because its the output with no resistor connected. My mistake.

Need to calculate the input devices.

Found a lot of information about resistors in parallel. Trying to learn as i go, thats why i am trying to do the math even if i don't have to. Just for fun :D

OK, re-read my own instructable (been awhile) to see you're trying to calculate Zload (the impedance seen by, say, an iPod connected as an input). Again, you don't really need to calculate this, but ...

The formula I gave is Zload = R plus (R/N)//Zin

(grrr... instructables Rich Editor removes the plus sign, so i have to write 'plus' in it's place)

NOTE: the notation '//' is shorthand for parallel resistance calculation. If you have 2 resistors in parallel, R1 and R2, the equivalent resistance is:

R1//R2 = R1*R2/(R1 plus R2)

So in your example

You're trying to calculate 1000+(1000/1)250*5 = 1251000 but as you noticed, this is way off.

Hey, just wanted to let you know I built one of these based entirely on your guide. I plan to eventually fill the box with epoxy or something but it works perfectly as-is! Thanks for a great Instructable!

Metal gives you shielding against stray noise, but if you're mixing headphone outputs then shielding isn't that critical and you should be fine with plastic. It's only when you're mixing line-level signals (e.g. Un-amplified microphone outputs, or what cones out of RCA jacks on stereos, etc) that you really need good shielding.

First of all Rich, Great job on the write-up and your willingness to respond to comments. I came across this when trying to figure out how to add an additional aux input jack in my truck and I was so impressed that I signed up to the site to give you kudos and ask my own question.

I am currently building a console and am incorporating the docking station (pictured below) to charge my phone as well as connecting it to the aux jack but I want the wires to be run inside the console to keep it tidy looking. Ideally, I want an input jack inside the console but I would still like to keep the original jack below the radio available though too in case I have someone riding with that may not have an iphone and can then plug into that jack. I have done some searches but am only finding splitters for audio outputs (for 2 headphones) rather than 2 inputs and am wondering if what you did here would work for my application.

Also, I am new to all this electronic stuff and am just learning about capacitors, diodes, resistors, and such. I was hoping you might explain how this device works (eliminating interference between inputs or how it determines which device to use for the output signal)

I guess you could build the mixer (with several input ports) into your dock/console and allow guests to connect to the mixer/console. The output of the mixer would take up the input port on your radio. Unless you want to mod your car stereo I don't know what else you could do other than buy a car stereo with a rear 1/8" stereo input port (they do make them that way).

Regarding how/why this works, basically a small amount of resistance keeps the input sources from fighting each other. Why? A piece of wire has nearly zero resistance and if you were to place it across the terminals of a battery, a lot of current would try to move through the wire and "bad things" would happen. If you add enough resistance to the wire, it will limit the current and keep the bad thing from happening. Hard to explain it more simply than that -- does that help/make sense?

You probably have a bad solder joint or possibly a short. Make sure and check that your input and output cables work when connected directly between the source and the amplified speakers or whatever you're driving.

If you have a volt-ohm-meter ($5 at harbor freight) you could check the resistance between the input and output, that would give you some idea of where the problem lies. Sorry I can't be more help.

I have a question, is there some way i could make say two sound systems in two separate cars play in unison? The idea of an aux splitter is possible but i have no experience in sound or electrical.. Any suggestions? Thanks in advance

Hey DJAWB, yes you're correct, your question really has nothing to do with the mixer, but I'm truly curious (i.e. don't read this question with a harsh tone)-- why on earth would you need the two cars to get the audio at exactly the same time?

Without knowing anything about what you're trying to do, what fidelity level is needed (voice or music), whether the cars are close vs. across country, etc. it's hard to say.

If both drivers have smart phones and good data plans, some type of network streaming would probably make the most sense. I'd do some research on setting up an IP-radio server. Maybe you could just use skype on your mobile phones?

If the cars are very close together, you might be able to use some kind of wireless technology like an FM transmitter (might need to be hacked for more power), CB radio, bluetooth, wireless guitar transmitter/receiver, or maybe just power a wifi router in your car so the other car could connect without using 3G data..

A few years ago I made a headphone splitter resembling this project. It worked great for when you wanted to share music with your friends without bothering anyone else. Though I was thinking of one day having the feature to switch from "Splitter-Mixer" with the flip of a switch (or a small circuit that would sense it and switch automatically). Was also considering adding a small amplifier circuit to it with a max volume shunt, to prevent people from adjusting it to high and bursting their ear drums. I'll let you know if I ever come though with either one of the two ideas.

Hi piergap, please read through the instructable text and discussion, this is addressed many times. If you have questions after reading, let me know and I'll be glad to explain anything that's not clear. The short answer is when driving line-level devices like self-powered computer speakers, the loss is not a big deal. OTOH, when driving devices like normal headphones that require power, the circuit in this mixer will not work well -- there is no free lunch and you have to understand the limitations. In the right scenario, this is an extremely handy little device.

Most powered computer speakers have a 1/8" headphone jack on the front -- just use that, or just buy a headphone amp. There are tons of opamp mixer designs on the web, but by the time you buy all the pieces it's probably cheaper to just get a Behringer (or other cheap brand) pro audio mixer -- easily had for $50-$100 and easy to find used. Unless you just want to make something...

Yes, a volume drop is to be expected, as explained in the project text.

In the past, most folks who have complained about the volume drop didn't bother reading the instructable text and were trying to drive passive headphones or speakers, neither of which works very well. This is meant to drive a powered speaker, preamp or headphone amp which in most cases can more than restore the volume drop.

You don't say what your inputs and outputs are or what you're trying to do, so it's hard to help you.

This is an outstanding project and I modified it slightly for a larger tin, because I wanted to use it as a passive mixer for dynamic microphones in a home studio. So the jacks are mounted in the lid and labeled "In 1" through "In 4" and "Out". I've weighed it down with two bags of dollar-store marbles inside the tin, for stability.

I've had a problem with trying to connect 2 computers to 1 set of speakers. I was using an audio splitter to merge the audio, but when computer 1 played audio, it would mute the second computer for 30 seconds after the sound stopped from computer 1. I assumed this was because the audio from computer 1 was messing with computer 2 when it went through the splitter. I built this with 1k resistors, but the audio from computer 1 still gets sent to computer 2 as well as the speakers, causing the sound to cut out again. Any ideas? I added images of my mixer because I like my layout lol.

You know, I think some PCs (laptops in particular) try to be smart and detect what is plugged into the 1/8 stereo output jack. Often times you will get a pop up saying what has been detected. That's the only thing I can think of.

I would check your mixer first with 2 "dumb" sources to make sure everything is OK. If that checks out, then look at your sound settings to see if there's a way to turn the detection off. Good luck!

I don't understand why I can't find these in stores. I have a couple computers I need to merge audio so they all use my sound system without having to re plug everything when switching computers. I use an audio splitter but it causes problems like one computer cutting out when the first computer plays sound.

Yes to both questions. The CMOY is made to drive headphones and my little mixer is made to combine multiple inputs safely into one stereo output. If you're building your own CMOY, it might be possible to put both functions in one tin, but I don't know if there's space for any extra inputs in a CMOY. That would be nice though.

So I just built mine, and it can actually power my headphones decently without an amp, but the amp is still necessary, and my cmoy kit won't get here for a while. I used these 1k ohm 1/8 watt resistors and these 3.5mm jacks, both of which are from radioshack, a Altoids Cinnamon Chewing Gum tin, and some 24 AWG copper wire I found in my grandparents garage. I will post pictures tomorrow.

I just finished mine, easy with this great tutorial. However something isn't like it supposed to be: I have 1 TV tuner and 1 media player as input, and Bose PC speakers (2.1) as output. The TV tuner is fine, but when using the Media Player, every now and then (+/- 20 minutes) there is a short load noise 'tick' (like quickly connecting an electric guitar to an amplifier) after which the volume drops with 15%. Then after another +/- 20 minutes, again shortly the noise 'tick' is sounding and the volume is restored. This happens in cycles.

I don't speak Spanish but google translated your comment as: "and if you want to control the damn one of those audio inputs as hagosi no potentiometer. stupid"

There is a schematic in the FAQ with stereo potentiometers if you want volume control (5k ohm, audio taper) . Many devices (ipods, etc) have their own volume control and in that case it is best to build the simple, cheap version.

Here is a google translation back to Spanish, no idea if it makes sense or not:(i no hablan traducción española, por google):

Absolutely, yes. You can split signals without resistors, it's only in combining sources that you need resistors between channels. Just put in an additional output connector(s) and wire it/them straight to the original output (i.e in parallel, no resistors between them). You could also buy a commercial headphone splitter and use that. Good luck.

Thank you for laying this out. It makes perfect sense. Question - could the 1/8" jacks be replaced with xlr plugs for microphones? Obviously the altoids tin would no longer be a suitable host. But, would the wiring still work out?

It would need some mods and some ground-rules or assumptions to be met. For the simplest case:

All the inputs are at a similar mic-level

All the inputs are balanced (a.k.a. differential, which xlr mic signals are).

None of the mics require 48vdc phantom power to operate (i.e. they're either dynamic or battery-powered condenser type)

In this case, you would want to lower the resistor values to maybe 50-100 ohms. Also, you would probably want variable resistance instead of fixed. For the original design, it assumes that each device has its own volume control, but microphones do not, so without a way to adjust the mix the mixer would be semi-useless.

It might be better to use an XLR-to-1/4" mono transformer/adapter for each input and build a mono version of the mixer with volume pots.

In reality, you can buy a decent XLR mixer for $50-100 new and by the time you buy the parts, it's probably not worth the effort, unless you really want to make your own.

I'd like to do 2 custom mixers, 1 for when I'm at friend(s) with my iPad (Skype, for teamplay with other friends at distance) and 360/PS3/Wii playing games. And a 2nd (for home) to combine the sound from PC (incl mic) and many other consoles. Why i'd like to do this is for one simple reason - convenience. It's way more convenient having the sound in the headphones instead of speakesr in which I'm not a fan of. I'd like to make a mixer with 1x3-poles input (PC/Console) 1x4-poles (iPad) and 1x3,5mm stereo output and 1x3,5mm mic input (leading to iPad mic in).

I understand there's a difference in volume levels in iPad/iPhone/handheld and PC/consoles, so how do i balance it out correctly and how do it know what resistors to use in order to max the volume in headphones? is pre-amp necessary? what to use?

The microphones will be tricky since they are getting DC bias voltage from the device (iPad,iPhone,PC,etc). Keeping the dc power sources separate, while still powering the mics and mixing the mic signals will be a pain and I'm not sure what it really buys you. It could be done, but I'm not very interested in designing it, sorry.

Honestly, I've built quite a few of these and never had any problems with shorts -- the resistor leads are rigid enough not to flex much on their own, even with a pretty decent shock. Even if there was a short, it would not cause any permanent damage.

But ... if you're the sort of person who stays awake at night worrying about this sort of thing, you could also try these alternatives:

'Potting' in electronics means to fill in the air space in the circuit enclosure with a suitable compound (non-conducting, non-corrosive, etc). Electronics-grade epoxy is used as a potting compound in military, automotive, aircraft, etc. This stuff is expensive and bullet-proof (figuratively speaking ;^)

I will be making this, though it'll be without the enclosure, as it will be for my multi-channel synth. Also. I recommend using a pcb. Theyre more difficult to do, but they keep your mixer from having a short. theyre are several PCB ibles out there.

Sorry for the delay in answering -- not 100% sure I understand what you're asking or trying to do, but I can make a few (hopefully) relevant comments that might help:

1) Even though this is technically a mixer, I almost never have more than one source playing at a time. Rather than make a complicated device to do switch automatically, I just play one device at a time. This gives the "clean" setup you mention with the added bonus that you can hear occasional notifications from a phone, iPad, GPS, etc.

2) Whether the audio comes from an 1/8", 1/4", RCA or whatever type of plug, you can substitute the required connectors and it should work fine.

The levels coming out of all devices should be similar, e.g. able to drive headphones. If you try to mix in a lower level signal (one that normally goes into a preamp) along with headphone level signals, you'll find that there will be a huge mismatch in the volume levels.

This is almost what I have been looking for. But I want to use headphones to listen to my GPS and MP3 player on my motorcycle. I've ordered 10 ohm and 20 ohm resistors and going to give it a shot. There is a 'commercial" motorcycle version for $179, or spend $500-700 on a new waterproof motorcycle GPS w MP3 player, and a $200 bluetooth helmet speaker system. I'll try this for $15 and see first....

Awesome instructable! Have mine made and it works. Question: Is it possible to prevent this from acting as a splitter? I am using mine to allow multiple hookups to a set of living room powered speakers... however, my home computer, one of the source, is also hooked up to the desktop powered speakers. When another source is playing through the living room, the altoids mixer acts as a splitter and sends the audio to the desktop speakers. Any way to prevent this?

The way I build the mixer there are 4 inputs and only one output, but you're talking about 2 output devices (livingroom and desktop speakers).

Normally, the outputs of all your sources would go to the inputs of the mixer and the output of the mixer would go to the input of the powered speakers. But that only accounts for signal going to one set of powered speakers.

How are the desktop/computer speakers even getting any signal at all?

If you can tell me exactly how things are connected I might be able to help you. A sketch/diagram is always easiest to understand.

I am only an amateur with electronics and soldering. I just wanted to say thanks for an easy and practical project. I have a KVM switch connecting my computers which does not support audio. The KVMs that do have audio do not work that well, in my opinion. Typically, they switch the audio and if you have a process running on another machine that is not in focus, the sounds are not played on the speakers. This is a great alternative!

I'm making a homemade stereo practice guitar amp out of some beefy powered PC speakers. I know that by itself, it doesn't have enough pre-amp to "work", but I plan on using it with a digital effects processor (Zoom pedal unit) 100% of the time. The Zoom has stereo headphone out with gain comparable to a headphone output on anything else. Tested and works great.

I haven't made the control panel yet, because what I'd really like to do is drop in an aux stereo input for hooking up an iPod to jam along with. It looks like I could pull this off just with an extra jack and four 1K resistors (two for the ZOOM side, and two for the iPod side). If I wanted to get fancy and add a mixing knob for the aux input, could I simply replace the aux-input's 1K resistors with a 1K stereo POT? Could it have any negative consequences for either device?

Great idea/design. I've been working on feeding 6 MP3 players into one output that would then feed an old PC speake amplifier with it's output going into my 100 amp radio with it's front stereo jacks. The reason for the amplifier is that the volume output of the MP3 players are not loud enough by themselves.

Do you see any problem in this combination such as the impedance of the PC amplifier, etc.? Would the 1 ohm resistors still be a good choice?

What you want to do is exactly the use case this mixer was designed for. You just need to add 2 additional inputs that are identical to the other 4 in the example. 1 KOhm resistors should be fine (not 1 Ohm, which I'm guessing was a typo?)

You should try feeding the output of the mixer straight into your car stereo input before even bothering with the additional stage in between. If this is an Aux input and is fairly high impedance (normal line-level input) you probably don't even need additional amplification.

By putting a diode in line with the signal, you are literally clipping it (everything below 0.3V will be clamped at 0.3V, called half-wave rectifying) and it's the same effect as when an amplifier is driven into overdrive and clips the signal. The smooth peak of the sine wave is replaced with a chopped-off square wave on the negative side which introduces harmonics and distortion.

I assume the reason you don't want to use resistors is because of the signal attenuation. Keep in mind that resistors are highly linear and introduce virtually no distortion, meaning the signal is still a clean copy of the original with a little less amplitude. Amplification can bring this back with a very high degree of fidelity to the original signal.

Diodes are non-linear and once the damage (clipping) to the signal has been done, you can't undo it later on. That's why I would never use diodes on an audio signal unless I was intentionally trying to distort it.

But hey, knock yourself out and try it. You won't hurt anything and maybe you won't be able to tell the difference. But I bet if you do compare the resistor and diode approach side-by-side, you will notice which one sounds better.

I would love to use this with my 4 wired lav mics (atr-3350 small battery powers it). Two questions - is it possible to add volume to each input? Would you need to power it? and what about a 3 way switch that would allow left/both/right for each? Would that be very hard to do?

Self powered lav mics would work fine. For volume control see circuit in FAQ section, step 15. power is only required if you add an active preamp circuit. Yes, you can add a left/right switch, I'd probably use a SPDT switch.

With a passive design you will always lose some volume. What are you trying to drive? A powered speaker? A passive speaker? Headphones? The lower the impedance of the load you're trying to drive, the bigger the volume loss will be.

Headphones. Actually, I am currently using it as a headphone splitter if it makes a differance, but the loss happens when I use it as a mixer also. I don't mind the loss of volume, I wouldn't put it much higher anyway, but I would like the possibility to though.

Yeah, if you read the notes and comments, this is not at all recommended as a headphone splitter. The impedance of headphones is in the 8-32 ohm range (quite low) and this mixer is not going to perform well unless your driving something with an input impedance of 1k ohm or above, like a typical powered speaker input.

For a headphone splitter you would be better off not using the resistors. The resistors are only required for mixing multiple sources together.

im not an expert in electronics and i would like to use this with headphones w/o amplifying it.... what if i lower the resistors to lets say 500 ohm or lower? would that work. Im trying to have 2 3.5 input sound run to 1 pair of headphones. i know if i used a headphone splitter to COMBINE 2 sound sources, and i turn BOTH volumns all the way up, it may damage the equipment. what are you thoughts of doing this?

In order to run headphones, you'd have to go lower than 500 ohms. Most headphones have an impedance of between 16-32 ohms and typically you want the source (i.e. amplifier) to have a lower impedance than the load (i.e. headphones).

If you want to experiment (at your own risk) and as long as you don't take the resistor values below the resistance of a typical set of headphones (again, 16-32 ohms), then you shouldn't cause any damage to either players or headphones and you can gauge the sound quality/volume trade-offs for yourself.

what sort of resistors would i need for a powered headset?http://www.roccat.org/Products/Gaming-Sound/ROCCAT-Kave-5-1/ is the headset, im guessing 10KOhm as it says on the tech specs but im not sure. im looking to fuse together the 5.1 3.5mm outputs on both my PC and a decoder box from my xbox, and also the headset signal from the xbox controller through the fronts.

1 kOhm would still be fine. You typically want the impedance of the source (mixer output) to be much higher than the impedance of the load (headphone). It's good that you're using a powered headphone. Interesting headphones, do you plan to mix 2 sources for each of the 5.1 channels? In other words you'll have 8 (10?) inputs and 4 outputs?

i have a tin that hopfully ill be able to put 4 different circuits in, 1 for each channel. yeah ill have 9 inputs in total as ill have an extra input on the front speakers from the output of my xbox controller, on a adaptor i have for it.

Hi there.I am currently working on alittle electrical project that I've put off for months due to the fact of not exactly knowing what I'm doing and I was wondering if you could help me out. I saw your project you made with the "Altoids tin audio mixer" on Make Magazine awhile ago and I had thought it would make a good headphone splitter if you reversed the way the audio was going into. (Basically one input feeding 4 outputs.) The problem is that I want to add alittle LED indicator light that automatically turns on when you plug some thing in and I don't know how to exactly wire it so that it could do that. Another question I had is do you still need the resistors if I make it into a headphone splitter instead of a mixer and could it be backwards compatible, like say I wanted to use it as a mixer one day instead of a splitter? I was possibly wondering if there was a way to amplify the signal that wasn't to costly (but if you can't help me out with that it's ok.) If you could answer my questions I'd really appreciate it.Thanks.

1) Regarding the LED -- Yes, in theory this could be done but I don't see the point. The project is entirely passive, so there are no batteries to either A) power an LED or B) die and require replacements or require anything to be switched off.

If you added an LED, a battery would have to be added and would only serve to light the LED.

If you're dead set on adding an LED, google on 'LED indicator circuits' or something like that and you'll find loads of information and circuits online.

2) regarding splitting vs mixing -- the ideal construction of a splitter is to use no resistors, while the ideal construction for a mixer requires resistors.

If you plan to split a signal to drive several high-impedance inputs (e.g. powered speaker inputs) then turning the mixer (with resistors) into a splitter will not be a problem. If you're trying to drive several pairs of headphones, then you will not want the resistors.

Cheap headphone splitters are readily available, while cheap, small mixers are not.

3) Regarding amplification -- I've pointed out several times in previous answers that you could easily add this mixer circuit a front-end to a CMOY headphone amp, which also happens to be built in an Altoids tin. Google on 'CMOY headphone amp' and you'll find tons of info.

Sorry for being a little late, but I can't seem to find the problem with mine. I can only get it to play out of the right speaker and it is only about half volume. I did everything that the instructable said and I can't find any shorts. There are only two things I can find that might be an issue. 1) I used different terminals. The link for them are here: http://www.radioshack.com/product/index.jsp?productId=21034522) There is one solder point near the output jack where my solder point on the output jack and my solder point where I soldered the 2 resisters together accidently got soldered together (So basically, what should be two solder points are now one). I hope that was a thorough explanation. Also, I am confident that it is the output jack which is the problem, because I tried all of the inputs and they are all working the same. Thanks for any information you may have.

It doesn't make any difference if you have 2, 3, 4 ... n inputs ... you can only drive a high-impedance device very effectively with the output of the mixer.

If you are driving an input to a powered speaker, aux input, etc this is no problem (these are examples of high impedance inputs). If you're trying to drive a headphone, speaker, etc (examples of low-impedance inputs) you are not going to be very happy with the sound level.

I meant to say that speakers, etc were *low* impedance devices ... meaning they require some electrical power to drive them. High impedance devices can accept an input with a very tiny amount of electrical power.

Assuming you have 2 stereo inputs, you would first need to mix each stereo channel down to a mono signal. Do this by tying the L and R channels together with 2 resistors in series (1K or so each), then feed the middle tap of the resistors to the L or R output mix.

I have a couple of police scanners in my car and it can sometimes sound like they're trying to talk over one another. However, if one came out of the left and one out of the right, the overlap might not seem so bad. That aside, you lost me almost right off the bat. I'm very novice but managed to understand your instructable pretty well, I think. But I don't get what you mean by tying the L and R channels together "in series." Sorry, it's been a long time since I did any kind of electronics project.

OK, interesting -- I bet the left/right separation would probably help you keep them apart better in your mind.

Yeah, I might not have explained what I meant in the clearest way. I'll try an ascii art picture and if that doesn't work maybe I can draw a real schematic. Here's what I was talking about. This is assuming the input is coming from the left:

You would tie the mono mix of one input to the L output mix and the other to the R output mix. This is equivalent to panning one input hard left and one hard right.

If you wanted to get *really* fancy, you could implement adjustable panning (i.e. audio positioning) so you could adjust each input's position in the stereo mix. I'd have to think about this but it would probably require a stereo potentiometer on each input channel.

Try the simple case first though -- that will give you the largest separation and if that doesn't make the 2 conversations more intelligible, then there would be no point in adding adjustable panning.

I *think* I understand. Let me regurgitate and see if I'm right: I have two input jacks. Each input jack has a left and right channel. I have one output jack, also with a left and right channel. I'm going to attach a resistor to the left channel of one of the inputs, and a resistor to the right channel of the same input, and join them together at the left channel of the output jack. With the other input, I'm going to attach a resistor to the left channel, a resistor to the right channel, and join them together at the right channel of the output jack. Let's see how artistic I can be:

You could do this in an altoids box, but I would probably just buy a 1/8" to 1/8" stereo cable (which you'll cut in half) and an additional 1/8" stereo solderable plug (or inline jack) from Radio Shack...

Cut the cable in half and solder the 2 halves up to the output plug or jack per the diagram above. Obviously, solder the grounds/shields together.

If you use an inline jack (like this: http://www.radioshack.com/product/index.jsp?productId=2104044) you will need an additional 1/8-to-1/8 stereo cable, but then you can use as long or short a cable as you need.

It's quick and dirty, small, cheap, no resistors and dead simple -- it's what I'd do in your situation.

This is great! I remembered seeing this forever ago and looked it up again today, have one hooked up with three channels so that I can listen to music or a call on my cell phone, notifications on my laptop and my two way radio while I'm at my desk at work.

I assume you meant to say 'signals' vs. 'singles' and I don't know if you mean input signals or output signals

I'll assume you're asking if you drive multiple output signals -- read the article and be aware that you can't drive headphones or speakers with this device, but driving two or more powered speakers, mixers, etc with high-impedance inputs, is no problem.

It's always helpful if you can be more precise in asking this type of question.

Hi, In this case you will have to turn the MP3 player off manually. Keep in mind, this is just a simple little device intended to safely mix several headphone outputs together into one output. Automatic muting and other features would be nice, but also add a lot of complexity, expense, etc.

Hmmm... sounds like a very interesting project (with a potentially annoying sound :) ). Do you mind if I ask what the goal is? Sounds like you're experimenting with analog synthesizer principles? Trying to drive away rodents? (sorry ;^)

Putting that aside, yes, if you have 3 existing devices that output the sounds you state above, you could definitely mix them and feed the output into a PC sound card line-in/mic input. Because the circuit is passive it is very quiet electronically and well shielded.

An easier way to prototype and experiment with this type of thing would be to get Audacity from http://audacity.sourceforge.net/ . Audacity is a free, open-source wave file editor (runs on PC, Mac and Linux).

In audacity, you can create multiple tracks which can be either recorded or tones generated by the program. Then you can mix them to your heart's content and even see (and hear of course) what the resulting waveforms look like.

So if your A, B and C devices already exist, you could record them each to a mono track and mix them. If they don't exist, you could generate the tones. You would probably have to do the rhythms manually by inserting silences every so often. Audacity is not really that sophisticated on the tone generation side, but there are tons of free synthesis applications out there.

My apology that I seemed exaggerate something~. Goal is the same to what you targeted. I have car-navigator of which speaker is out of order anymore so that I need to connect it to the car AUX input together with homemade karaoke system.

I already have voice-pass filter circuit for my mp3 player together with echo-mic connected without resistors added like your device. The mixed sound without resistors is not good as you know. This time my goal is nothing but to save some money for commercial mixer.

I just wanna check ur device. A, B and C are pure sounds of Piano key A (440Hz beep sound), B and C. By feeding pure beeps within narrow frequency bends(440Hz +or- 2Hz? , 493.9Hz and 523.3Hz === each one row or line of frequency bend), I would like to see how accurately your mixer could make pure 3 rows of lines. If your device is not as good as commercial mixer ( i don't know how good the commercial mixer can make the mixture, either), the result frequency spectrum would not look like 3 rows but distorted or noise induced lines, that is, I'd rather have to spend some money to buy commercial mixer to enjoy good sound in my car.

Thanks for Audacity. Chk the frequency analyzer here: http://relisoft.com/Freeware/index.htm . You can SEE the sound in spectral display. Simply say "Do, Re, Mi, Fa, Sol, Ra, Si, Do, etc...". In the spectral frequency display, you could see the frequency bends used in making each sound. If you provide pure note A sound(440Hz), it will make pure horizontal line near 440Hz height in the display. I will build your device and will see the result of the mixture of 3 feedings.

For simplicity, I designed this assuming that each device has its own volume control.

But if you wanted to add a volume control to each channel, I'd do something like this, shown in the first picture. Each channel has 1 stereo pot, 10k audio taper and a resistor (in the 4.7-10k range):

If you wanted to have one master volume control, do something like the second picture. The single pot is also a stereo potentiometer, about 10k with an audio taper.

I don't think you'd have a good result trying to put individual channel controls *and* a master volume control without going to active opamp circuitry because you'lll start to lose too much signal at that point.

richfiddler11- Favorited this 'ible not only because of the project but because it's one of the best written/presented I've seen in awhile (including your thorough comment replies)! Two comments, neither meant as critical :-)

1) Too bad you couldn't figure out how to work a Dremel into this build, then you could have entered the contest!

2) OK, this may be nothing but semantics, but: until you talk about adding the pots & tapers etc. for volume/input gain control, isn't this really (in an audio production terminology sense) a patch bay, not a mixer? *sardonic grin*

Good point about the Dremel tool (hmmm... is it too late to add one? ;^) and thanks for the nice words.

On the semantics of what we call this thing, we could always agree to disagree (boring) but I've always been a sucker for a good friendly debate, so I'll tell you why I'm standing firm with the name "passive stereo mixer". Then if you still don't agree, you can tell me how you see it as a patch bay -- deal?

OK, here goes: With the device here, we're taking signals from multiple sources and combining them in a safe way into a single, composite signal. That's more or less the definition of a mixer (see http://en.wikipedia.org/wiki/Electronic_mixer for a good discussion and note that the schematic in the article is virtually identical to the circuit here). Stereo comes from having 2 channels, and passive comes from using only passive components and no amplification => Passive Stereo Mixer.

A patchbay, on the other hand is another type of passive device that can be used for several purposes:

route a bunch of inputs to a bunch of possible outputs using short audio cables (kind of like an old-fashioned telephone switchboard)

provide a convenient way to insert signal processing effects into a signal path without having to unplug wires

Your instruction is unbelievably wonderful and simple for what I wanted to make.

I have a question. See my concept. There're 3 channels (mono). Ch#1 has only one pitch with various beats or rhythms. Note = A 440Hz.Ch#2 also has only one note B(493.9Hz) with various and random beats.Ch#3 also has only one note C(523.3Hz) with continuous beeping or random beats.Each ch doesn't include any noise at all originally.

Put Ch#1#2#3 into your mixer. And connect PC's REC input with your mixer's output to record the mixture of Ch#1#2#3.

What the .wav's spectral frequency display looks like? I expect very sharp 3 rows of solid or dotted lines. Is it right? I would like to know if your device can make noise-proof mixed result.

Thanks, there's no electronic reason for suggesting that the resistor body is close to the input jack. There are 2 reasons I said that:

1) makes it a little easier to connect all the output sides of the resistors together in 1 "solid" wire.

2) I consciously decided not to include a schematic in this project, since it is so easy to build and it might intimidate some folks. So I thought positioning the resistors that way would keep some builders from getting confused about which side the next resistor should connect to.

Being an EE myself, I appreciate what you're saying: a schematic is the most concise way to describe a circuit and in your place as a consumer of someone else's how-to (which I frequently am) I would also prefer a schematic to a blow-by-blow account.

As the writer in this case, I choose an approach I *thought* would be accessible to a wider range of instructables.com readers. At the same time, I did put a schematic diagram in the FAQ section for the cognoscenti -- did you see that? Maybe it should have been under a more obvious heading like "Schematic", huh?

Ah, I have all the parts handy but the tin. I didn't quite understand until I saw the schematic. Now I'll tackle it with confidence. Probably just stick it on a wooden ruler and make sure everything is grounded. Thanks for the reply, Dan Derrick

You have to understand that the bare copper wire and the case are already connected together electrically on purpose. So it doesn't make a bit of difference whether the wire touches the case randomly or not.

The ground tabs of all the jacks are tied together through the case and you could almost get away without the copper wire. However, a good solid ground is critical for quiet audio circuits and the connection between the jack and the case is not reliable, so the copper wire is there to provide a rock solid return path.

OTOH, if any of the resistor leads touches the case, you will definitely have a problem.

quite interesting, pretty surprised there isnt really a built in amp powered by a battery to "power" the out put, usually being speakers. then again, not sure if those inputs are powered or not, are they coming from outputs?

I built something like this with 4.7k resistors, and I'm finding the volume to be a little too low for my amp. Do you think that going back and refitting the mixer with 1k resistors would give me a noticeable volume boost? If it isn't much, I don't want to bother, but if it will keep me from having to set my amp to the highest level, I wouldn't mind going back for Stereo Mixer 2.0

It will make a difference in volume, but I don't know if it will be enough to make it worth your while.

If you want to try an experiment, tack solder a 1k ohm resistor in parallel with one of your existing 4.7k resistors (best to do this for both right and left channels). The resulting parallel resistance is about 820 ohms. If you tack a 1.2k ohm resistor in parallel, you will get closer to the 1k ohm mark (955 ohms).

Now you can compare the volume level between the 4.7k input and the 1k input.

If you have the small leads with alligator clips on both ends you could also just clip the resistors in parallel instead of tack soldering.

I went back and resoldered 1k resistors to my project. I found that the volume was reasonably louder, however, there was a significant drop in volume with each additional input I connected. I decided that I could live without 2 extra inputs, so now I have a total of 2 inputs mixed together per output.

My project has two outputs (one for front speakers and one for rear), and I have two inputs mixed for each out. For my front speakers I'm mixing my Home Theater-Front and my laptop audio, and my rear speakers are mixing my Home Theater-Rear and my iPod-couch hook up.

I don't think I tested different number of inputs with the 4.7k resistors very thoroughly. What I remember is that the 4.7k caused the volume to drop significantly regardless of how many inputs were connected, the number did not affect the volume drop much. With a 1k resistor, the volume doesn't drop as much with one or two inputs, but each additional input after that causes volume to go down quite a bit.

Ideally, I would have loved to have been able to connect more than 2 inputs passively, but I don't know enough about how to make that work without incurring significant volume drops. The reason that this volume drop is a problem is because my Home Theater outputs include a center channel that isn't being filtered through a resistor, and I don't want the front channel significantly louder than the other speakers which are being filtered. I suppose that I could add another output to my mixer to include the center channel, but that seems a little unnecessary as no other audio is ever going to be routed to that speaker.

Anyway, if you happen to know a way to mix 3 or more inputs without affecting the volume greatly, please let me know.

I bought a pile of these jacks from ebay, and they have the extra feature of a normally closed (NC) switch in both the left and right channels that opens when the plug is inserted -- this adds 2 extra tabs.

This type of jack is used in devices where the speakers get shut off when the headphones are plugged in and it saves the manufacturer the cost of an extra switch. It's a nice feature if you need it, but for this application the built-in switches are totally unnecessary.

In fact, the extra tabs just get in the way so I (very carefully) figured out which ones I didn't need and snipped them flush.

Since there's more than enough detail for most folks to absorb here, I didn't really want to confuse anyone by going into that particular discussion.

Just built this with 2 inputs last night (mainly because I ran out of 4.7k ohm resistors) and I am very impressed! Works great! I would suggest using a dremel tool to clean up the edges of the drilled holes (using proper safety protocols of course).

If you intend to only have one audio input "active" at any given time, would distortion still be an issue or not? I'd like to do this for my PC desktop speakers so I can switch multiple audio sources to the same speakers without having to unplug/replug the 1/8" speaker cord every time. In this use case, only one device would have a signal on it at any point, so are resistors necessary?

It doesn't make any difference at all if the other inputs are active or quiet, you still need the resistors.

Here's why: When you tie several outputs together directly, each one "sees" the others as a low-impedance load, and what's worse, each additional device you add lowers the total load impedance seen by each device (parallel resistances or impedances divide).

Why is this bad? The lower the load impedance, the more power the device's output must try to provide. That's where distortion and/or destruction come in.

hey, i am trying to make one of these but have a few questions about my logical uses of it. I want to take output from my laptop and my mp3 player and play it through my koss pro/4AA headphones. Also, would it be possible to use it as a splitter without any modifications from the mixer?

Recently, I sat down and figured out the exact formula for the impedance each input device sees (see the FAQ). Based on that analysis, I now think that you could get away with a significantly lower resistor value, like 1k ohm, maybe even 600 ohms. Also, I used to be worried about the number of parallel inputs lowering the load impedance, but now realize that the resistor value and the input impedance of the amplifier are the dominant terms. So using a 600 ohm or 1k resistor will help reduce the voltage drop, and hence the volume drop, and should not result in too much distortion from the input devices. Other than that, you'll have to resort to active circuitry like the LM386 or an op-amp to get more gain -- no free lunch! Look at building a CMOY headphone amp with this mixer as a front end maybe...

Hi, I'm not sure if you're referring to resistor power rating (1/4 watt and 1/8 watt) or stereo plug diameter (1/4" and 1/8") -- luckily, in either case there would be no need to change the resistor value. Also not clear on what you mean exactly by 'channel switch' -- that could mean input channel selector, on-off switches for each input channel, or other. Please look through the comments below as folks have already asked about a bunch of different variants -- after that, if you have a question that hasn't been covered, please let me know. Thanks, Rich

I must have done somethnig wrong... I made it as described except I used 1/2 watt 4.7K resistors instead of 1/4 watt. When I plugged my CD player into the "input" the output was very faint. There was a large drop in volume. Has anyone else had this happen?

This circuit will attenuate the signal, no doubt about it. I've never found it to be a problem, but of course you do have to turn up the volume on the amp and your signal sources more than you're probably used to.

You don't say what type of amp you're feeding and what the input impedance might be. (Don't even think about trying to drive a set of heaphones.)

Keep in mind that the lower the impedance of the device you're driving (i.e. the amplified speakers, car stereo aux input etc), the bigger the signal loss will be. Higher input impedance is better in this case.

Also, is this CD player output a headphone jack, or RCA jacks like you see on a stereo component? I've never tried connecting pure line-level stereo components together with the mixer, and I can imagine the output would be rather faint.

Hi, Google on 'CMOY headphone amplifier'. There are all kinds of plans and kits out there. You could put a 2-channel mixer in front of the headphone amp circuit and do what you're after. Good Luck, Rich

Great project, Rich... I'm getting ready to throw a modified version of this together and have a couple of questions. 1) is this design safely expandable to say, 8 inputs? 2) I'd like to have 2 switchable outputs (each going to different sets of amplified speakers). Anything special required for this other than a couple of dpst switches at the outputs?

First off, if you're going to pot the thing ('potting' = filling up the air space with something non-conductive to make it more rugged), make sure everything is working beforehand! Once this stuff is in there, all repairs are off.

Personally, I don't think it's necessary -- yes, it will definitely make the mixer more rugged, but is a little overkill IMO.

OK, that being said, I did a bit of research and here's some second-hand information that I've read, but can't confirm from personal experience:

- GE Silicone II (available at any home-improvement store) supposedly does not leach acetic acid, so it would be OK.

- automotive stores carry a grade of RTV that is "oxygen-sensor safe" and may be called 'type-b'.

- some people think that the acetic acid is no big deal, so use any RTV you want (these same folks probably still use asbestos potholders in their kitchens, and think that Radon gas and global warming are conspiracies, but there you go).

Thanks dude, you are THE man! I wanted to ruggedize it a bit because i could easily see myself sitting on the handy new device, or dropping it or something. its tough having to idiot-proof all my gizmos

Great Instructable, very well written. I have a question though... What I want to do is basically the opposite of what you have here, except with a different quantity: I want to have one input and two outputs, so I can have my computer plugged into both my headphones and speakers, with a switch that switches between the headphones and speakers as outputs. That eliminates the need to crawl under my desk to change what is plugged in back there. So... a) Could I build ^that^ with your mixer, but with the addition of a switch? b) If so, where would I put the switch, and what would I connect it to? You might even make an entirely separate Instructable about this...

If you want to just switch between two inputs you would need to buy a DPDT switch (double pole, double throw). Tie all the grounds together, connect the L&R inputs to the center taps and the two outputs to the outer poles of the switch.

Radio Shack used to make something like this, model 12-2315, but it's now discontinued. RS closed them out a $5 a while ago but you might be lucky enough to find one in a dusty corner of a RS, or on ebay.

Here's a plan I found while googling on the 12-2315 where somebody hacks a 12-2315 unit to make a basic passive mixer like we have here:

Is there any chance that I would be able to replace the switch with a cross-fader, and if so, would I need a ridiculously larger box or could it remain remotely close to your original design? Or could I, just have two inputs and a 2 channel fader for each input, that way there is a bit less wiring and components that the four input version? If it is at all possible I'll do it, even if it's a bit pricey. Thanks.

I added a page to the Altoid Tin mixer project to cover some of the FAQ's that came up from comments here. There are also some additional design comments in case you want to experiment with lower resistor values, different number of inputs,etc.

After looking at the circuit more carefully, I now think it would probably be OK to use resistors in the 1 or 2k ohm range. This would result in a little less volume loss.

Thanks for all the good & interesting comments as well as positive ratings!

I've always thought a mixer where one channel could override another would be useful (for example, have your mp3 player playing on one channel feeding into your car stereo, and when your GPS starts talking, have it override the music). Can you think of a way to do this? Transistors? Diodes?

What you're describing is called "Ducking" and the only commercial mixers I've seen with that feature are made specially for restaurants, etc. so when they call your name for a table, it automatically turns down the music.

I can't think of a way to do it with only passive components, but that's not saying it can't be done.

With active circuitry, you could have the GPS channel feed a detector circuit that would turn on a FET transistor when the GPS was talking. The FET would be set up to shunt (i.e. short) a mix of the other channels to ground and the detector could be made from a diode and capacitor.

Who knows, maybe the detector could generate enough voltage to turn on the FET without needing an external battery?

I'll have to think about that...

P.S. as long as you're not totally cranking the music, the circuit as built works OK with the talking GPS.

Yeah, I thought active components might have to play a role. Brilliant idea with using the transistor to shunt the other channels to ground, though. It occurs to me that if you are routing music and GPS through there and amplifying it afterward, it doesn't matter how loud the music is because the GPS' audio output will be amplified just as much.

Any idea how this can be done as a DIY project or if there are any good DIYs already out there? I've looked for quite a while and haven't found one. I'm pretty handy, but I'm not an EE, but I can follow instructions. ;)

I have a diagram that came from Kitplanes magazine several years ago. It's not a passive setup, but it does use very easy to acquire parts, probably all in stock at Radio Shack or your favorite local electronics store.

This looks like a basic opamp mixer except there is no feedback resistor on the opamp, which is kind of strange. Also, is there supposed to be a ducking circuit in there? What is this supposed to do related to an RC plane?

Kitplanes magazine is for people who build their own aircraft (home built aircraft), not for RC models. There was a guy named Jim Wier who wrote their column on "Aero'lectrics". This is where it came from. He still maintains a website here: http://www.rst-engr.com/.

The resistors don't alter the sound much at all, in fact much, much less than circuits with active components like transistors and op-amps.

You do lose a *slight* bit of gain due to the resistors, but without the resistors you run into distortion and/or damage to the headphone driver circuits of your expensive iPods, etc due to overloading. Also, the slight gain loss is not a problem because most amplifiers have more than enough gain to make up for it.

Regarding resistor value, honestly you'll be just fine with anything between 4.7k ohms and 10k ohm range. Much lower and you'll start overloading and distorting your headphone/line outputs. Much higher and you'll start losing gain excessively with no real advantage. For the cleanest sound, go for the 10k value.

Unfortunately, volume loss is in the nature of this circuit. In my experience, my car stereo has more than enough gain to make up the loss and it never has bothered me.

Is your car stereo not able to make up the volume difference? Also how high do you have each individual device's volume set?

Lower resistor values will result in less volume loss but more distortion, so you have some room to experiment.

Keep in mind that the number of inputs plays a role here as well. The lowest impedance seen by any of the input devices (which drives the distortion level) is roughly equal to the resistor value divided by the number of inputs (because they're parallel circuits).

Zload = R/num_inputs

With the design as stated here, the Zload value is about 1.1k ohms. In reality the input impedance of the car stereo will be in parallel with the load value as well.

So with only 3 inputs, 3.3k resistors would give you a similar value and result in lower volume loss. You can make those kind of tradeoffs if you want.

You're right and this project is designed to plug into an input with fairly high impedance, say 2k ohms and up. This happens to be the typical input of powered speakers, amplifier line-ins, and car stereo aux-inputs, so all is good.

I've been using these little mixers for several years and can say that the design works very well.

Do you think it would be possible to turn this into an automatic switcher instead of a mixer? I'm in the same boat as you -- I want to connect my iPod and talking GPS -- but I think it'd be even more useful if it could sense signal from (for instance) the GPS and switch to it, instead of mixing it in with the music, since it'd probably be hard to hear the instructions over the music.

I'm not an EE or anything, so I have no idea if this can be done passively..

Not sure what you mean by powered input here -- this is really designed for audio line-level signals and the output of an amplifier would probably fry the mixer.

For mixing a guitar or bass with music from an mp3 player, you could always run the bass through an effects processor or practice headphone amp like the C-Tech Rock-it, Pod mini, or the like -- then feed the practice headphone amp output into the Altoids tin mixer.

Could a slide switch be added to select which input is piped to the output? Your idea for using it with multiple computers sounds like an attractive option. I've got 3 systems plus and iPod and my cell phone. I was thinking it would be great if I could pick any individual input or all of them.

I was interested in making this, but with two RCA type plugs for the output. Would adding these change the wiring at all? Or would I just connect one line of resistors to one plug, the other line of resistors to the second plug, and connect the grounds together? Thanks!

I have a broken mainboard which had a 6 channel audio card, mow that the mainboard is boken. I want to use the 6 head stereo jack from the mainboard.

I have a few questions about this device.

1. Can i do it without using the resistors? 2. Can this device be used to attach microphone to pc? 3. Can this device be used in reverse, i mean for connecting audio input (or mic if possible at all) to many speakers of line in jacks?

2) You could definitely go to a mono design and use 1/4" jacks. There is even a commercial design based on this idea, google on 'DOD AC240'. It would work OK for instruments that have a strong output, like electric guitar, keyboard, acoustic guitar with built-in preamp, etc.

It wouldn't be a *great* mixer, and commercial mixers from companies like Behringer have gotten very cheap and good. Still, it could be handy for some situations.

I was not infact wishing to do the opposite, it was purely for interests sake, however, if it were the case that headphone splitters were cheaper than the components, couldn't you buy a headphone splitter cable and modify it to produce the same outcome as this, at a lower cost?

OK, sorry, I didn't get what you were driving at. Yes you could do what you're suggesting.

I've found that commercially made cables like this can be tricky to strip and solder, since the wires tend be as small as possible. Also, it's nice having the electrical shielding of the Altoids tin, but I'd say give it a try.

Why wouldn't you use diodes instead of resistors. With a diode wired in parallel to each jack you would prevent the "back feed" in the unit without adding resistance. Or, if you did that, would there be too much power coming out of the unit?

Unfortunately, diodes really won't work here. Diodes only allow current flow in one direction and audio is an AC signal that needs to flow in both directions.

A diode as an isolator, like you suggest, will clip half of the signal, which amounts to really bad distortion.

Also, even in the direction that current does flow, no current flows until the forward voltage across the diode reaches 0.7V, so you end up losing part of your signal that way as well -- more distortion.

What you're suggesting works well in DC circuits though, like batteries in parallel, etc.

Could you explain, or point to a reference explaining, what you mean by "sonically transparent" please? I haven't been able to find an adequate explanation through google or on wikipedia.

Like ERCCRE123, I too am wondering about the purpose and necessity of the resistors. If the mixer-input-jacks are from devices intended to be plugged directly into a stereo (in your case car) input jack, doesn't it stand to reason that they could be safely passed directly to the mixer-output-jack?

If the intention of the resistors is to safeguard devices from each other, then diodes would likely be the proper choice.

Hi, great questions, I'll break them into parts and try to explain clearly without oversimplifying:

Regarding "Sonically Transparent", this is a popular marketing term, but here it means that the resistors do not have any frequency dependency so the circuit passes all audio frequencies equally. Also, no distortion is introduced by this passive circuit.

Understand that the more load you attach to an output driver, the more power is required of it, and the more distortion is present. Lowering impedance = increasing load and vice versa.

Your iPod can drive a pair of 16 ohm headphones (low impedance, high load), no problem, but at significantly higher distortion than when it's driving a 5000 ohm line audio input (high impedance, low load). It can probably survive an outright dead short (very low impedance, very high load) but why go there?

Output drivers themselves have very low impedance, meaning they can source and sink lots of power. When you connect 2 output drivers directly together, they are each now presented with a low impedance load (i.e. each other) and the best thing that can happen is distorted sound. The worst is overheating and destroying one or both of the amp circuits. Again, I'm not going to design a circuit that hopes for distorted sound at best!

Bottom line: we're trying to get the best sound/least distortion from each audio device, which means loading the output as little as possible. The cost is a little loss in gain, but it happens that most modern amps have enough excess gain to drive you deaf, so this is a good trade-off IMO.

Putting diodes in place of the resistors would give you some pretty crazy sounds and in fact, diodes are the basis of many electric guitar distortion effects.

Resistors are linear components, meaning they can change a signals amplitude, but not its frequency.

Diodes are non-linear components, which change frequency content of an input signal.

Diodes are great for protecting DC batteries attached in parallel, but need to be used carefully in an audio path.

Good question, and it would certainly be simpler to get rid of those annoying resistors, but unfortunately you really do need them.

Here's what happens if you tie the headphone/line outputs of two or more devices together without resistors: the outputs will overload each other and "fight" each other, resulting in distortion and possibly damage to the output driver circuits.

I don't think you really want to destroy an expensive iPod or cell phone, so please bite the bullet and use the resistors if you build this project.